This invention belongs to field of electronic components and more specifically into multifunctional and/or multipurpose electronic elements. Such elements provide simultaneous and independent exploitation of several electric properties, which are result of activity of various physical mechanisms, in common material structure. Even more narrow definition places this invention in the field of polycrystalline semiconductor diodes.
Diode of this invention differs from standard planar monocrystalline Si based diodes, as it provides higher capacitance with stable temperature and wide range of frequency properties, which enables the diode to perform condenser function. Capacitance of this newly disclosed diode may be controllable altered, by the means of voltage, which provides similar function as known for varactor diodes (word varactor is developed from variable reactor).
Ixe2x80x94U characteristic of the subject diode is not linear and a pulse mode breakthrough is not destructive, which provides similar function as known for Zener protective diodes.
A method of manufacturing a multilayer ZnO polycrystalline diode that protects against electrostatic discharges, over-current, and voltage surges overcoming the aforementioned drawbacks is provided. The present invention further includes preparing a plurality of oxide additives and homogenizing the oxide additives with ZnO. A plurality of polycrystalline layers is then formed having a thickness of approximately 20 to 60 microns. The method further includes forming a plurality of polycrystalline layers, each having a thickness of approximately 20 to 60 microns, printing an inner electrode on each polycrystalline layer, forming a compact block with the polycrystalline layers, cutting the compact block into a plurality of chips, scorching and sintering the plurality of chips, forming a plurality of outer electrodes, scorching the outer electrodes, and placing at least two of the outer electrodes on each chip.
There are four basic problems, from aspects of use of planar Silicon (Si) diodes as protective elements:
1. In voltage range v less than 4V Si diodes have high leakage current (reverse current) and high negative breakthrough coefficient, which excludes Si diodes from above voltage range, as protective elements.
2. Energy absorption capability of Si protective diodes, at higher voltage, is very limited. This is due the planar construction and small volume of reduced Pxe2x80x94N area, where all absorbed energy is concentrated, respectively. Any increase of absorption capability of diodes automatically claims increase of their dimensions, and as result price, which is not in accordance to demands of contemporary electronic and microelectronic.
3. In many various applications, especially when voltage stroke and frequency disturbance appear simultaneous, the protective element shall have capacitance as high as possible. The capacitance of Si Zener diodes is usually small therefore normally condenser is parallely added.
4. Use of Si planar diodes as SMD elements, due their construction, demands mounting in special plastic housings, which additionally increases their dimensions and price.
Technical problem solved with this invention arises from special and general demands of electronics for contemporary protective components.
a) Special Demands
1. Widest possible operating voltage rang of diode
2. Non-destructive and temperature independent breakthrough
3. Relatively high capacitance, stable in wide temperature and frequency range
4. Controllable change of capacitance by means of voltage in whole range of operating voltage
b) General Demands
1. Ilimitability of small and large dimensions
2. Choice and change of electrical parameters in wide range
3. Possibility of surface mounting
Pxe2x80x94N junction and potential barrier on it respectively represents base of functioning of the most elementary active semiconductor two terminal component diode. The most commonly used material for manufacturing semiconductor diodes is silicon monocrystalline. Selective and controlled forming of N, N+ type areas respectively, and P, P+ type respectively is achieved with diffusion or ionic implantation in such manner that the most of formed Pxe2x80x94N junction is located in monocrystaline depth, where the surface remains even, thus planar (planar technology). Depending on doping profile, geometry, and polarization conditions, diode offers various useful functions for different types of applications.
Breakthrough Ixe2x80x94V characteristic of planar diodes is very non-linear, which provides use of diodes as protective elements against electrostatic discharge and voltage strokes. Namely, at lower voltages, through diode current is very low however, when electric field on Pxe2x80x94N junction reaches certain value (about 106V/cm) diode resistance rapidly decreases and very high current runs through the diode. Such diode behaviour is caused by two different mechanisms: a) tunnel effect; and b) avalanche like multiplication, where the value of breakthrough voltage mostly depends on width of reduced area and dope level respectively of N and P area. Both mechanisms are often very actively used in various fields of application, one of the fields being protection.
Tunnel effect is known for Si diodes, with breakthrough voltage vp,4Eg/q (Eg-semiconductor suppressed band energy, q-elementary charge). As Eg with increasing temperature decreases, such diodes have expressly negative temperature coefficient. Further more, leakage current of such diodes in breakthrough area is very high, which disables them to function as protective elements, especially at DC electronic circuits. If breakthrough voltage Vp greater than 6Eg/q than avalanche like multiplication is basic breakthrough mechanism. In such case breakthrough voltage is positive and breakthrough voltage is increasing with temperature. If diode breakthrough voltage 4Eg/q less than Vp less than 6Eg/q, both breakthrough mechanisms function at the same time.
Regardless to the fact that capacitance, as result of reduced area of Pxe2x80x94N junction, is one of its basic features, diodes are exceptionally used as condenser, however mostly in special cases. This is due to low dielectric constant of silicon (xcex5=11.9), low value of surface capacitance ( less than 10 nF/cm2) and due to limits of planar technology, which are also related to price.
Wideness and capacitance of reduced area is directly related to its voltage. This is utilised in varactor diode, where capacitance may be controllable altered with voltage. Because of these properties varactor diodes are widely used in parameter boosters, harmonisation generators, signal mixing systems, detectors and as voltage altered trimmers for precise setting of resonance frequency. Beside high sensitivity (s greater than 3) of temporary varactor diodes with hyper gradient junction in some applications they show deficiency, e.g. relatively low value of nominal capacitance, large dimensions and incapacity of surface mounting. It is obvious that possibilities of planar Si technology are limited, when all demands of contemporary protective electronic components must be fulfilled. This is especially case when necessary for the component to have the highest possible capacitance (e.g. frequency disturbance filtration) or when very high and frequent energy loads are present (electricity in automobiles).
Some of above mentioned problems could be solved using new materials and structures. European patent EP 41 8394A of Matsushita Company, introduced multilayer condenser based on SrTiO3 semiconductor, which has varistor characteristic. Because of high value of xcex5 (i.e. greater than 15000), such condenser provides very high capacity, varistor function being worse side of this product. Namely in prebreakthrough area varistor characteristic has high leakage current, thus highest operating DC value must 50% lower, compared to varistor breakthrough. Further more non-linearity coefficient a does not exceed value 15, which makes efficiency of the protection very limited. U.S. Pat. No. 4,811,164 introduces condenserxe2x80x94varistor, monolith multilayer component, composed from two different materials in such manner, that one multilayer package is composed from dielectric ceramics based on BaTiO3, having xcex5 greater than 10000 and is closely bonded with second multilayer package, made of ZnO varistor ceramics. Both packages are sintered together and from monolith chip component, composed of two different materials, which both maintain their basic condenser and varistor properties, thus they combine these properties in one component. U.S. Pat. No. 4,729,058 introduces self-limiting multilayer condenser, based on multi compound system composed of oxides of the following elements: Zn, alkali metals, B, Co, Si, Bi in Al. In that patent authors T. K. Gupta and W. D. Straub discus influence of design and mutual disposition of internal electrodes to capacitance and resistance and breakthrough voltage of self-limiting condenser.
Subject of present invention is multiplayer zinc oxide diode, which is multifunctional component. Multifunctionality of diode of present invention comes out from fact, that beside symmetrical and very non-linear Ixe2x80x94V characteristic of ZnO the diode has stable and optional capacitance ranging from some ten pF to some ten nF. Capacitance of ZnO diode may be controllable altered in range from 10% to 30%, with voltage.
Key for understanding of operating and construction of ZnO semiconductor diode is in understanding physics and electrical properties of ZnO surface and border line between two ZnO grains, which is also the smallest possible ZnO diode and elementary element for ZnO polycrystalline diode.
ZnO is semiconductor of type N, with free carrier concentration nxe2x89xa11016 cmxe2x88x923 and wideness of suppressed band Eg=3.35 eV. Bi2O3 is basically weak dielectric. These two materials are elemental compositions of ZnO diode. Namely MIS diode (Metal Insulator Semiconductor), with structure as shown in FIG. 1, has extremely non-linear Ixe2x80x94V characteristic (xcex1=60-100) in inverse polarization (1) as shown in FIG. 2, the breakthrough voltage value depending on Bi2O3 thickness. In direct polarization (2) Ixe2x80x94U characteristic shows much lower non-linear characteristic. Bi2O3 is know as one of the best ionic conductors, which is confirmed by its Ixe2x80x94U characteristic (3) in FIG. 2. When Bi2O3 is doped with above elements, e.g. Co or rare earth elements, such as Y, Pr, Nb etc., ionic conductivity of Bi2O3 decreases on account of increased gap conductivity. In both bases electronic conductivity is very low. Thus MIS diode of structure Bi2O3xe2x80x94ZnO, with inverse polarisation provides good non-linear Ixe2x80x94V characteristic.
Cxe2x80x94V characteristic of such diode is shown in FIG. 3. Cxe2x80x94V relation proves existence of negative charge on intersurface of Bi2O3xe2x80x94ZnO and existence of reduced area (xe2x89xa1100 nm) on the ZnO side and existence of potential barrier on the border line between Bi2O3xe2x80x94ZnO, which is restored to maintain electro neutrality on the border between two systems. Therefore at lower voltage, through current will be low and as a result of thermal ionic emission over the barrier also temperature dependent. Further increase of inverse voltage on MIS diode will result in surface inversion, i.e. hole concentration (minority carriers in ZnO) becomes higher than concentration of electrons. Namely, inverse voltage is divided between dielectric and reduced area, arid electrical field on top of the barrier could reach value about 2 MV/cm. Under these circumstances the electrons coming over the barrier will gain enough kinetic energy (they become xe2x80x9chotxe2x80x9d) to initiate shock ionization and start creating electron-hole pairs. Intersurface gaps compensate part of negative charge and instantly reduce potential barrier, which enables rapid increase of current, i.e. breakthrough.
Voltage at which an inversion is reached, (i.e. breakthrough starting point) is called threshold. Threshold value VT is from 3.3 and 3.8 V and above all it is function of the Bi2O3 thickness, charge concentration on intersurface and free carrier concentration in ZnO. Thus not much depending on temperature (i.e. temperature coefficient has small negative value), as experimentally confirmed.
To create symmetric, very non-linear Ixe2x80x94V characteristic, which would be independent from direction of polarisation, symmetric SIS structure (Semiconductor Insulator Semiconductor) as shown in FIG. 4. In such case, both dielectrics on one and another side form potential barriers with equal properties. Thus, regardless to the direction of polarization, one of MIS eiodes is inverse polarized, which provides non-linear Ixe2x80x94U characteristic in both directions, as shown in FIG. 5. It is obvious, that conductivity of such diodes, influenced by voltage changes rapidly, from very low (at V less than VT) up to very high (at Vxe2x89xa7VT) and that in both areas, different conducting mechanisms exist.
Elements having such characteristic (i.e. variable conductivity) may be designated as VARICON (VARiable CONductivity).
Equivalent electric scheme of ZnO diode is shown in FIG. 6, where RZno is ZnO resistance, Cixe2x80x94dielectric capacitance, RDxe2x80x94reduced area resistance and CDxe2x80x94reduced area capacitance. Thus serial connection of number n of such diodes will increase breakthrough voltage of diode equivalent, which may be calculated as Vn=nVT. On the other hand, parallel connection of number n of such diodes increases capacitance of diode equivalent, which may be calculated as CN=nCD.
A great number of such serial and parallel connections of elementary ZnO diodes are found in polycrystalline system ZnOxe2x80x94Bi2O3. At heat treatment sintering of dust mixture of these two materials polycrystalline structure is formed, which is composed from grown ZnO grains and surrounding phase of Bi2O3. Balance layer Bi2O3 with thickness of 2-10 nm is always formed between two ZnO grains. Thickness of this layer does not dependent on beginning concentration of Bi2O3, or way of processing. This means that structure of borderline between two grains is repeatable in entire volume of polycrystalline material, which provides great number of primitive ZnO diodes having very similar properties. Due to improvement of electric properties of mentioned materials it is necessary to add some other oxide materials, such as: Mn, Co, Nb, etc., which are Bi2O3 conductivity modifiers, and regulators of concentration and distribution of surface condition on the border line of ZnO; Fe as concentration regulator of free carriers in ZnO; Sb as regulator of uniformity and ZnO grain growth speed thus porosity regulator of polycrystalline material.
Optimal ZnO diode forming structure having preferred properties is multilayer structure in from of chip (4) as shown in FIG. 7. Such structure is composed from a number of equally thin (15-300 xcexcm) polycrystalline layers (5) among which thin (5-15 xcexcm) metal layers (Ag, Pd, Pt, mixtures or alloys thereof in various proportions) are placedxe2x80x94inner electrodes (6), (7) in such manner, that each second electrode is shifted in opposite direction, so only one, the smallest edge ends in the smallest lateral side of the chip. Other edges of inner electrodes are surrounded with polycrystalline material. All inner electrodes ending on one side of the chip (6) are mutually connected with metal layer, which represents outer electrode (8). Similar description may be applied for all inner electrodes, ending on the opposite side of the chip (7). Outer electrodes on both lateral sides of the chip (8), (9) provide mutual electrical connection between every second inner electrodes in such manner, that between outer electrodes of the chip, parallel connection of ZnO polycrystalline diodes, represented by thin polycrystalline layer (5) with corresponding electrodes on each side (6) and (7) exists. All polycrystalline and inner metal layers together form compact low porous monolith structure. Basic electrical parameters (i.e. breakthrough voltage and capacitance of ZnO diode) may be influenced by parameters of multilayer structure.
Breakthrough voltage VN depends on ZnO grains number in the cross section of the polycrystalline layer between two adjacent inner electrodes and its thickness respectively. Combination of ZnO grains dimension and layer thickness may provide breakthrough voltage in range from 4 V to 150 V. Capacitance CN depends on dielectric constant of polycrystalline material, in our case varying in range xcex5=900-1400, on thickens and number of polycrystalline layers in the structure. Therefore it is possible to provide capacitance in range from 100 pF-100 nF with real chip dimensions from 1 mm to 10 mm. Capacitance linearly decreases with increasing of DC voltage in whole operating range of voltage. This provides controllable altering from 10% to 30%.